Very bad.
The screenwriters were incredibly lazy. They literally copied everything from Coben's book, which is set in New Jersey, not UK. For example, no one in UK would say "private school", everyone would say "public school". There are a million details like that.
Then there are things that simply go beyond stupid - for example, the main character talking to his dead fiancé in the middle of an AA meeting without anyone noticing he is talking to himself. Or main character's MRI scan showing "degenerative brain disease" but thirty minutes later all that damage is suddenly not there anymore and it's medication that caused his seizures. Or the witness who was lying below deck on a boat drunk as a skunk but somehow he heard the killer going up to the deck and well he didn't actually see one brother pushing another but "as soon as he heard the splash he knew". Oh and another thing - that witness goes to the insane asylum for treatments once a year and yet everyone - Maya, the police...take his words at face value and believe them like they are gospel. Or Maya flying a combat mission and literally moving the hand in front of her co-pilot's face to deliberately switch off the comms and the co-pilot inexplicably sees nothing so Maya just says "oh the comms are down" and that's it.
There are so many moronic things like these and the whole things feels like a teenager trying to come up with ideas for a school play and then says oh I can write a whodunit for a change - because this script has the quality of high school kid's first attempt at writing.